1. Technical Field
The present invention relates generally to a method of forming conductive structures on a substrate and more particularly, relates to forming conductive structures by injection molded solder.
2. Related Art
In the semiconductor industry, it is conventional to generate electrically conductive structures on a semiconductor chip packaging substrate such as wire bond interconnects for packaging, and other passive elements such as inductors, capacitors, coils, transformers, baluns (i.e., a transformer for matching an unbalanced line to a balanced load), antennae and redistributions (i.e., another layer of interconnect for signals or power lines).
Injection molded soldering (IMS) is a flip chip bumping technology developed to reduce wafer bumping costs by reducing process steps. See, for example, U.S. Pat. No. 6,133,633 to Berger et al., which illustrates one approach for building interconnect structures using IMS. The process of IMS melts bulk material, usually solder, and dispenses it onto a wafer-sized mold which is coefficient of thermal expansion (CTE) matched to the device substrate. The mold is scanned with the molten material and thereafter cooled so that the material solidifies. An illustrative mold 10 is shown in FIG. 1. It can then be inspected and aligned to the substrate. The aligned assembly is then heated in order to transfer the bumps from the mold to the substrate. When the material is in a liquid state and the corresponding structures are oxide free, the wetting forces exceed the surface tension forces that maintain the molten material in the mold. After cooling to solidify the material it is released from the mold as it is removed from the substrate. The structures retain the shape of the mold. The molds are reusable.
One shortcoming of the above-described IMS approach, however, is that it is limited to low aspect ratio interconnects, i.e., with an aspect ratio of less than 3:1. For example, as shown in FIG. 1, the openings in mold 10 have a low aspect ratio. Ideally, passive elements should be generated with as high an aspect ratio as possible to provide the highest performance with the minimal space usage.
In view of the foregoing, there is a need in the art for a way to form conductive structures on a substrate that does not suffer from the problems of the related art.